Fossils put a new face on the ancestors of Neandertals

Neandertals came into the world face first. Or at least, their lineage did, according to Spanish paleoanthropologists who analyzed 17 ancient skulls from a deep bone pit in the Atapuerca Mountains of northern Spain. The facial bones and teeth of these people, who lived 430,000 years ago, already resemble those of Neandertals, which are known from much younger fossils. Yet the Sima people also still had relatively small brains and other primitive features, suggesting they were very early members of the lineage that eventually gave rise to Neandertals. The analysis offers a detailed look at the murky origins of our closest cousins and has implications for the evolution of key traits such as brain size.

Researchers have long debated when and where Neandertals arose. Neandertals stem from the same root as our own ancestors, but the two lineages parted ways sometime in the past 500,000 years or so. Modern humans arose in Africa at least 200,000 years ago, whereas the fossils of Neandertals are found only in Europe and Asia after 230,000 years ago. Between about 200,000 and a million years ago, our view of human origins is blurred—most of the fossils of hominins, or members of the human family, are isolated, fragmentary, or spread widely across Europe, Asia, and Africa.

The 12-meter-deep Sima de los Huesos (“pit of bones”) at Atapuerca offers the largest trove of hominin fossils ever found, including bones from 28 individuals. Many researchers expected these ancient hominins to represent the common ancestor of both Neandertals and modern humans—a species called Homo heidelbergensis, which lived about 500,000 to more than 1 million years ago.

But the skulls from Sima de los Huesos challenge that view. Seven new skulls, plus 10 others deposited in the pit at about the same time, have distinct Neandertal traits, including robust lower jaws, small teeth at the rear of the jaw, and thick brow ridges with a distinctive double arch. This means they are the “oldest reliably dated” fossils of proto-Neandertals, according to the report in Science by lead author Juan Luis Arsuaga, a paleoanthropologist at the Complutense University of Madrid, and his colleagues.

The presence of distinctly Neandertal facial features suggests that these traits were among the first unique characteristics that the species evolved. Thus, they offer a glimpse of the way that small differences in chewing, for example, can lead to key anatomical changes and send a group of humans down a new evolutionary path.

In addition, if these small-brained Sima fossils are indeed ancestral only to Neandertals, then our cousins must have evolved their big brains independently from us. Rather than inheriting big brains from a common ancestor, Neandertals and modern humans each developed that trait on their own, perhaps favored by changes in climate, environment, or tool use experienced separately by the two species “more than half a million years of separate evolution,” writes Jean-Jacques Hublin, a paleoanthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, in a commentary in Science.

The Atapuerca team also redated the Sima de los Huesos site using several different methods. At 430,000 years old, the fossils are probably too young to belong to H. heidelbergensis, the team argues. And yet they are already more Neandertal-like than the type specimen for H. heidelbergensis found in Germany, which lacks Neandertal traits in its teeth seen in the Sima fossils. The Atapuerca team suggests that the bones be reclassified as a new, still unnamed species that was the immediate ancestor of Neandertals, but not modern humans.

Paleoanthropologist Chris Stringer of the Natural History Museum in London agrees that the Sima fossils are ancestral only to Neandertals, not modern humans, but questions the date and classification of the find. He thinks the fossils may be younger and suggests simply calling them early or archaic Neandertals, rather than giving them a new name. But he and others familiar with the finds agree: European Neandertals had already developed their own distinct look by 430,000 years ago.